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The Crab Nebula: standard candle no more?

12 January 2011

Teaming up with other telescopes monitoring the Crab Nebula, ESA's INTEGRAL observatory has made a significant contribution to demonstrating that this source, previously believed to be a standard candle, might not be so reliable, after all. The small, but measurable dimming of what was until now considered to be one of the brightest and, most importantly, the steadiest source in the high-energy sky calls for a re-examination of how X-ray and gamma-ray observations are calibrated.

The Crab Nebula is probably the most iconic relic of a star's demise: first spotted as a supernova in 1054 AD, this object appears today as the remnant of that explosion, a vast nebula hosting a rapidly spinning neutron star, or pulsar, at its core. Furthermore, it has long been notable for being one of the brightest sources in the hard X-ray and gamma-ray sky and an extremely steady source as well, its flux varying by only the tiniest amount. These characteristics led astronomers to regard the Crab Nebula as a standard candle, up to the point that fluxes in high-energy astrophysics are customarily measured in units of the Crab's flux.

For decades, this emblematic nebula has been employed to set the absolute flux scale of the instruments on board most of the space-based telescopes that observe the Universe at the highest energies. Instrument calibration is a complex matter, and even more so in the hostile environment of space, but since the earliest days of X-ray astronomy—the 1960s—the Crab Nebula has been employed by astronomers as an indisputable normalisation ruler. That is, until recently, when, in June 2010, an examination of nearly two years of data gathered by NASA's Fermi Gamma-Ray Burst Monitor (GBM) revealed, for the first time, evidence for a possible dimming of this supposedly steady source.

"When we first saw the decline of the Crab Nebula's flux in the Fermi GBM data, we thought it must have something to do with the instrument, because the models that are widely accepted and used in the astrophysics community predict that the Crab is a constant source," comments Colleen Wilson-Hodge from NASA's Marshall Space Flight Center, who led a study that has confirmed that this nebula is not the standard candle it had been believed to be. "In order to rule out any possible instrumental effect, we decided to gather data from as many telescopes as possible that had been routinely monitoring the Crab Nebula over the past two years, and we were astonished as we discovered that all instruments agreed: the dimming is, unexpectedly, due to the nebula itself," she adds.

Besides Fermi GBM data, the extensive data set put together by Wilson-Hodge's team comprises observations performed with a number of world-class high-energy telescopes: ESA's International Gamma-Ray Astrophysics Laboratory (INTEGRAL), and NASA's Swift Burst Alert Telescope and Rossi X-Ray Timing Explorer (RXTE). This comprehensive study, based on instruments that rely on very different observing techniques, was crucial to confirm a real, intrinsic decline of the Crab Nebula's flux of about 7% in the 15–50 keV energy band (corresponding to wavelengths between 0.008 and 0.002 nanometres), and a similar decline in the 50–100 keV band (0.002–0.001 nanometres).

The data show that the decline arises from the nebula, and not from the pulsar lying within it, as no unexpected variations have been detected in the pulsed flux. A possible physical interpretation for the dimming is a change either in the acceleration mechanism of the electrons that make up the nebula or in the nebula's magnetic field. Irrespective of the cause, this surprising result's resonance goes well beyond learning more about the nature of the Crab Nebula, and is likely to have huge implications for the entire field of high-energy astronomy.

"Until recently, the Crab Nebula was the one source in the high-energy sky that was considered to be consistently both bright and steady," notes co-author Erik Kuulkers from ESA's INTEGRAL Science Operations Centre in Spain. "The observed dimming as described in our paper, and other recent and historic reports of very short-term flares at very high gamma-ray energies as seen, for example, with AGILE and Fermi LAT, make us question its general role as a reliable standard candle. X-ray and gamma-ray astronomers feel as if the rug has been pulled from under our feet," he adds.

This result could not have been achieved with a single telescope: the key to the study led by Wilson-Hodge lies in the complementary nature of observations made with a series of instruments covering different, but overlapping energy ranges, from 2 to 500 keV. The contribution of ESA's INTEGRAL observatory was instrumental in ruling out the possibility that the observed dimming was an orbital effect. "Unlike the other spacecraft involved in the study, which are all on relatively low orbits, INTEGRAL has a highly eccentric orbit, meaning that it can perform observations while being above the van Allen radiation belts that are present around the Earth," comments Chris Winkler, INTEGRAL Project Scientist. "Therefore, the detectors on board INTEGRAL operate in an environment that has a different level of background radiation with respect to the other spacecraft, and this allows us to exclude any orbital induced background effect in the Crab Nebula's dimming," he explains.

Without a standard candle, it will become necessary to rethink the way most X-ray and gamma-ray observations are performed, although it is not yet clear how. Some telescopes, among them INTEGRAL, used to regularly point to the Crab Nebula for calibration. "We will probably still be able to use the Crab as a calibrator, but we will need to constantly monitor this source in order to carefully characterise its variations and take them properly into account when studying other objects," says Kuulkers.

It is also unclear, at the moment, whether the dimming will continue indefinitely, or whether the high-energy emission from the nebula will rise again in the future. "We are all eager to see the outcome of future observations of the Crab Nebula and to figure out what is causing this standard candle to fade and flicker," concludes Wilson-Hodge.

Notes for editors

The study is based on data collected using NASA's Fermi Gamma-Ray Burst Monitor (Fermi/GBM), Swift Burst Alert Telescope (Swift/BAT), and Rossi X-ray Timing Explorer Proportional Counter Array (RXTE/PCA) as well as two instruments on board ESA's INTEGRAL observatory: the Imager on Board the INTEGRAL Satellite (IBIS) and the Joint European X-Ray Monitor (JEM-X).

The orbit of ESA's INTEGRAL satellite is a highly eccentric one, with high perigee which provides long periods of uninterrupted observations with nearly constant background and away from trapped radiation (electron and proton radiation belts). The initial orbital parameters were a 72-hour orbit with an inclination of 56 degrees, a height of perigee of 9,000 km and a height of apogee of 150,000 km. The orbit evolves with time due to solar and lunar gravitational disturbances such that current values for these parameters are: a perigee of about 3,800 km, apogee of 146,200 km and inclination of 80 degrees.

The Crab Nebula, also referred to as M1, NGC 1952, and Taurus A in various catalogues of astronomical sources, is one of the brightest sources in the high-energy sky. With a flux of about 1.43 × 10-8 erg/s/cm2, this object is significantly brighter than most sources, whose fluxes are mostly a factor of 10 or more lower.